This invention relates to a process for the production of polyvinyl chloride and the use thereof for the manufacture of soft foam materials. More particularly, this invention relates to the use of an improved emulsifier system in such a process.
It is known to produce emulsifier-containing polyvinyl chloride (PVC) by continuous polymerization in emulsion. This process is carried out in the presence of water-soluble catalysts and generally in the presence of ionic emulsifiers such as alkyl sulfonates, alkylaryl sulfonates, alkyl sulfates, alkali salts of fatty acids, etc.
The thus-prepared polymer dispersion is generally worked up by spray-drying, forming finely grained powders which can be processed with plasticizers to pasty compositions capable of being cast, i.e., the so-called pastes or plastisols. Various processing methods are known for plastisols, e.g., casting processes such as rotational casting, dipping processes and spreading processes. Of particular interest is the manufacture of soft polyvinyl chloride foam in accordance with the spread-coating method. In this process, the plastisol paste can be combined with a blowing agent, e.g., azodicarbonamide. At the high temperatures, generally of 160.degree.-240.degree. C., in the gelling tunnel, the blowing agent is decomposed; the paste is foamed and simultaneously gelled, and a soft foam material is thus produced. This process has broad fields of application, for example in the manufacture of imitation leather. The chemical literature describes this spread-coating method for producing soft polyvinyl chloride from, e.g. in Kunststoffe, 52. Annual, (1962), Vol. 10, page 624-629.
In this spread-coating process, two properties of the spread-on foam are of great importance. On the one hand, the viscosity of the plastisol paste is desirably maximally low before gelling, i.e., as shown in example 1 to 5, determining the viscosities by means of a viscometer "Rotovisko RV 1" of the firm Haake with a measuring head 500 for a shearing range of 5-400 sec.sup.-1 and a measuring head 50 with speed reducer 10 for a shearing range of 0.3-16 sec.sup.-1. On the other hand, there is the requirement that even with a minimally brief residence time, e.g., of about 1.3 minutes, in the gelling tunnel a thoroughly foamed product be obtained having a fine, uniform pore structure, a low density and a smooth surface. Heretofore, it has been impossible to satisfy both demands simultaneously.
In the conventional utilization of alkyl sulfonates as the emulsifier in the production of polyvinyl chloride capable of plastisol formation (e.g., as described in U.S. Pat. No. 2,727,627), pastes are obtained having a satisfactory viscosity characteristic; however, it is impossible to produce a foam material with short residence times in the gelling tunnel, as can be seen from Example 5.
If alkyl sulfate is used conventionally as the emulsifier during the polymerization (e.g., as in French Pat. No. 977,296), a foam having satisfactory properties is formed from the paste prepared from this polymer even at brief residence times of 1.3 minutes, but the viscosity properties of the paste are unacceptable, as can be seen from the results set forth in Example 6.